The present disclosure relates to power amplification circuits. A power amplification circuit is used in a mobile communication device such as a cellular phone in order to amplify the power of a radio frequency (RF) signal to be transmitted to a base station. A bias circuit is used in such a power amplification circuit to supply a bias voltage or bias current to an amplifier. For example, an emitter-follower bias circuit in which a heterojunction bipolar transistor is used is disclosed in “Evolution of Power Amplifier for mobile applications” Satoshi Tanaka, International Meeting for Future of Electron Devices, Kansai (IMFEDK), 2013 IEEE, pp 112-113.
In a power amplification circuit, a bias current may increase and the output current of an amplifier may increase as the input power increases. The output characteristic of such an amplifier changes depending on the characteristics of the bias circuit (output impedance etc. of bias circuit, for example). Regarding this point, in the configuration disclosed in the above-cited document, since the characteristics of the bias circuit are fixed, the output characteristic of the amplifier is also fixed. Therefore, in the configuration disclosed in the above-cited document, the output characteristic of the amplifier cannot be freely adjusted.